Guardians of the Galaxy director James Gunn confirmed the news on his Facebook linking to the Marvel.com announcement.

Yep it is completely official. AND he's amazing. I never knew someone could say I AM GROOT so many different ways and with such feeling (and that sounds like I'm kidding but I'm not!)

Gunn also commented about the delay in announcing Diesel's involvement:

Because he wasn't officially attached until very recently. He wanted to do it and we wanted him to do it but some kinks needed to be worked out.

The following image was also released:

The "Guardians of the Galaxy movie" has an August 1, 2014 release directed by James Gunn and starring Chris Pratt as Peter Quill Star-Lord, Dave Bautista as Drax The Destroyer and Zoe Saldana as Gamora, Benicio Del Toro as The Collector, Michael Rooker as Yondu, Karen Gillan as Nebula, Lee Pace as Ronan, Djimon Hounsu as Korath the Pursuer, with John C. Reilly as Rhomann Dey, Glen Close as Nova Prime, and Bradley Cooper as the voice of Rocket Raccoon.